Why does the Torah say "l'Zar'acha Nasati Es ha'Aretz ha'Zos" (in the past tense), even though they did not yet possess it?
Rashi: Because the promise of HaSh-m is considered as if it has already been carried out.
The first two times (Bereishis 12:7; 13:15), He used the future tense. Gur Aryeh explains similarly - the above verse (12:7) was a conditional gift, dependent upon the conduct of the recipient. Here, HaSh-m made a covenant to Avraham, meaning the gift would certainly be fulfilled. According to Gur Aryeh, this was the entire reason for the covenant; refer to 15:6:0.3:3; also refer to 15:10:1:3.
The River Peras was the last of the four rivers mentioned in Bereishis 2:14. Why then is it called, "ha'Nahar ha'Gadol"?
Rashi Because it is attached to Eretz Yisrael. 1
In keeping with the saying, "The servant of a king is a king; cleave to a ruler and the people will bow down to you!"
What was the extent of HaSh-m's promise to Avraham's children?
Targum Yonasan: HaSh-m promised him that his children would not go to Gehinom, and that He would redeem them from exile and give them Eretz Yisrael. 1
Ramban: Refer to 15:18:4:1.
Opinions differ in the Midrash as to whether HaSh-m revealed to Avraham [the events that would befall his descendants in] this world, or even the World-to-Come; and whether He revealed "ha'Yom ha'Zeh" or even "ba'Yom ha'Hu" (as alluded to in the verse). What does this mean? Why did HaSh-m show all of this to Avraham?
Maharal (Gevuros Hashem Ch. 8, p. 50): At this juncture of the Bris Bein ha'Besarim, HaSh-m showed Avraham all that would emerge from him. Everything stems from Avraham's merit; he began the building of the world [and was the archetype and root of the Jewish people who would follow]. Opinions differ as to whether Olam ha'Ba is also included here to Avraham's credit. "Ha'Yom ha'Hu" refers to the times of Mashi'ach.
This is the third time HaSh-m promised Avraham that He would give his children Eretz Yisrael. Why three times?
Ramban: The first time (Bereishis 12:7), He promised him the land where he was, but without specifying the borders. The second time (Bereishis 13:14), when Avraham's merits had increased, He added "north, south, east and west," which meant either literally in all directions or that He showed him the borders of the land (like He did to Moshe Rabeinu), and it would belong to his children forever. Whereas here (the third time), He defined the borders and clinched it with a Bris, assuring him that, a) they would retain it even if they sinned; b) even if they sinned and had to be expelled, they would return; and c) that He would be their G-d - inasmuch as He personally, would govern them there and would not hand over control to any other star or Mazal. 1
QUESTIONS ON RASHI
Rashi writes: "[The past tense is used because] an utterance of HaSh-m is as if already carried out." Yet, in verse 17:8, the Torah will return to future tense, "I shall give to you and your children"!
Gur Aryeh: The later verse (17:8) refers to the actual giving of the Land, which would take place only in the future.